Personnel who work in environments in which exposure to either x-rays or nuclear radiation is possible are periodically monitored to determine if the radiation levels to which they have been exposed fall within established safety limits. In addition, environmental monitoring of radiation, as for example, ambient radiation levels in the vicinity of nuclear power plants, or background radiation resulting from naturally occurring sources, also requires the continuous monitoring over a period of time of low level radiation doses.
Monitoring of cumulative exposure to radiation is generally provided by a dosimeter. For monitoring of personnel, the dosimeter is usually configured in the shape of a small badge which may be clipped to a person's clothing and worn whenever there is a possibility of exposure to radiation. At periodic intervals, the dosimeter badges are collected and analyzed to quantitatively determine the amount of radiation exposure which they have accumulated during that time interval. Such monitoring is mandated by various governmental requirements for personnel working in nuclear power plants, radiology departments of hospitals, or in laboratories which utilize x-ray or nuclear radiation sources for experimental purposes.
By way of background, dosimeter badges have been developed in the prior art which utilize various detection materials. The simplest type of dosimeter is one which incorporates a small strip of photographic film within a light-tight enclosure. X-rays and/or other energetic nuclear radiations penetrate the enclosure to expose the film. The change in optical density of the film over the monitored time period is an indication of the total dose of radiation acquired by the film. Although film as a dosimeter material is inexpensive, it suffers from poor sensitivity at low dosages, and is of course not reusable after exposure.
To overcome these limitations, prior art dosimeters have also been developed which employ various types of solid state thermo-luminescent (TL) materials as the radiation dose accumulator. Irradiation of a TL material with x-rays or energetic nuclear radiation produces defect states in the material which trap electrical carriers generated therein. The number of trapped carriers is proportional to the total dose of radiation absorbed by the TL material over a period of time. To measure the accumulated dose, the TL material is heated to a temperature which releases the trapped carriers, causing them to produce characteristic luminescence (generally at infra-red wavelengths) which is optically monitored. The intensity of the luminescence is a measure of the total radiation dose which the TL material has received during the monitored time period. TL materials are generally more sensitive than photographic film to low doses of radiation, and can be reused after the TL material is heated.
To rapidly analyze the accumulated dosage of large numbers of thermo-luminescent dosimeter (TLD) badges, automatic analyzers or "readers" have been developed in the prior art. Such readers, for example, can make automatic measurements on five hundred TLD badges in three hours without manual operation. To quantitatively relate the measured thermo-luminescence intensity to the radiation dose, the sensitivity of the electronics in the readers must be calibrated against a "primary standard" dosimeter badge which has received an accurately known dose of radiation from a radiation source of known intensity.
Besides the need for overall calibration, in order to quantitatively analyze TLD badges, the reader must also properly correct for the variation in sensitivity between badges. Variations in sensitivity occur not only between different TL materials, but also as a result of variations in manufacture of the same TL material. Thus, prior to field use, each dosimeter is irradiated and measured against a standard which has been irradiated with the same dosage to determine a relative sensitivity factor for the dosimeter as compared to the standard. After the relative sensitivity is determined, the dosimeter is ready to be placed into field use. Upon subsequent analysis of the accumulated dose, the reader utilizes the relative sensitivity for the particular dosimeter being measured as a correction factor to determine the absolute dose.
In general, "primary standards", which have been irradiated with a precisely known dose of radiation are utilized to calibrate the electronics in the reader, and are handled carefully so that the properties of their TL materials do not change with storage. For more routine evaluation of relative sensitivities, so-called "secondary standards" may be used, since the absolute dose with which they are irradiated is not critical. All that is required is that the dosimeter whose relative sensitivity is being evaluated be irradiated to the same dose as the secondary standard.
The design of TLD badge readers is quite advanced. However, prior art irradiators for producing the requisite "primary standard" or "secondary standard" badges for calibration purposes are deficient from a number of practical perspectives. In one prior art irradiator, exemplified by a Model 142 irradiator manufactured by J. L. Shephard and Associates, dosimeters are manually positioned around the circumference of a large cylindrical housing having a radiation source (e.g. .sup.137 Cs) appropriately shielded in the center thereof. The radiation source is raised out of the shielded position to irradiate the badges with an accurately known dose of radiation. However, during irradiation the radiation source is completely unshielded, and as a consequence, personnel are not permitted to enter the room while the irradiator is operating. Although the open air design of this type of irradiator permits the production of "primary standards", the manual loading and unloading of dosimeters is time consuming, and the requirement that personnel cannot be within the same room during irradiation undesirably limits the type of location in which the irradiator can be installed.
A second type of prior art irradiator exists (manufactured by Williston-Elin Co. of South Africa) in which loading and irradiation of TLD badges are performed automatically. In this prior art irradiator, TLD badges are placed into a magazine capable of holding approximately fifty badges, which is then placed within a shielded tunnel. At the start of an irradiation cycle, the radiation source is raised from a shielded enclosure in which it is stored, and positioned in the middle of a shielded housing, where it remains until all the TLD badges in the magazine have been individually dosed. In this type of irradiator, mechanical assemblies similar in design to those used in automatic TLD badge readers sequentially raise each TLD badge out of its location in the magazine and into a position in close proximity to the radiation source, where it remains for a preset period of time. In this manner, each TLD badge is individually exposed to a controlled dose of radiation. Since the radiation source is at all times contained in a shielded housing, personnel can remain in the room during operation.
Although automating the movement of TLD badges into and out of a shielded housing, this type of prior art irradiator also suffers from a number of deficiencies. First, during irradiation, the TLD badge is positioned very close to the radiation source. This geometry is not at all optimal, and results in a non-uniform dosage across the TLD badge because the edges of the TLD badge, being somewhat farther from the source, receive less radiation than the central portion of the TLD badge.
Second, the TLD badge receives not only a known quantity of direct radiation from the radiation source, but also an unknown quantity of secondary radiation induced in the walls of the shielded housing by the radiation source and scattered in all directions. This scattered radiation generally consists of low energy x-rays which result from the interaction of the primary radiation emanating from the radiation source with the walls of the shielded housing. Lead shielding commonly used in the housing is particularly prone to generation of unwanted scattered radiation. The amount of scattered radiation may fluctuate with time, and is difficult to estimate in a quantitative manner. As a consequence, a TLD badge irradiated by this type of prior art irradiator cannot be used as a "primary standard", but only as a "secondary standard" for determining relative sensitivities.
A third deficiency in both types of prior art irradiators is that the radiation intensity incident on the TLD badges cannot be varied. This renders large dosage variations difficult to achieve in a time efficient manner. Variability of dosage is a feature many users consider to be important. For example, dosimeter applications involving environmental monitoring generally result in radiation doses which are approximately a factor of 10-100 times lower than the doses encountered during personnel monitoring. To achieve accurate dose measurements in the reader, it is necessary to provide calibrated standards dosed to approximately the same level as the dosimeters being monitored. Since there is no provision in prior art irradiators for attenuating the intensity of the radiation source, large variations in dosage can only be achieved by varying the irradiation exposure time over several orders of magnitude, or by physically changing the radiation source. Both of these approaches are undesirable.